This invention relates to a process assembly including the fixturing that is used to support an article in a process application, and, more particularly, to such fixturing made of an opencell ceramic solid foam.
Fixturing, also sometimes termed tooling, is required in many processes to support articles that are being processed. Typically, the fixture is positioned in relation to the supported article so as to retain spatial and dimensional relations. For example, the supported article may include two components that must be positioned precisely relative to each other during the course of the processing, and the fixture achieves and maintains that relation. In another example, the supported article includes only a single component that is supported over a wide area to prevent sagging and other dimensional changes during processing at elevated temperature. There are many other examples of the use of fixtures in manufacturing technology. Some fixtures are used only a single time, and other fixtures are designed to be reused many times before being refurbished or replaced.
The fixture for each particular manufacturing operation is often costly to produce, because it must correspond to exacting requirements of the supported article. Fixtures are required in order to produce operable final products in many cases. They are therefore an expensive but necessary part of the manufacturing operations.
High-temperature mechanical properties and wear resistance are often the limiting factors in the operability and life of a fixture. For a fixture that is to be used in elevated-temperature processing of the supported article, the fixture must retain sufficient strength at the elevated temperature so that the required geometrical relations are retained. For a fixture that is to be reused many times, the wearing action of the supported article on the fixture causes the dimensions of the fixture to change slightly, so that after several re-uses the required dimensional relations may no longer be retained.
Fixtures are made of a wide variety of materials according to the specific requirements of the supported article and the processing in which the fixtures are to be used. Many fixtures are made of metal because it is readily machined to shape, and some fixtures are made of other materials such as ceramic because it has good strength properties at elevated temperature and is wear resistant. Metal fixturing has the disadvantages that it loses strength at moderately elevated temperatures and often has low wear resistance. Available types of ceramic fixturing have the disadvantages that they are difficult and expensive to produce to exacting tolerances.
There is a need for an improved approach to the fabrication of fixturing, particularly used in process applications requiring heating to elevated temperatures. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for processing a supported article that requires support from a fixture during the processing, and particularly provides the fixture that is used in the processing. The fixture is fully compatible with a wide variety of manufacturing processes for a wide variety of supported articles. The fixture has excellent mechanical properties at elevated temperatures. There is little change of dimensions when the temperature of the fixture is changed over wide ranges. The fixture is also highly wear resistant, so that it may be used in multiple repetitions of the manufacturing operation. The fixture is readily prepared to precise tolerances by a relatively inexpensive procedure.
A method for processing a supported article that requires support from a fixture during the processing comprises the steps of fishing a sacrificial ceramic fixture precursor having a shaped portion thereof shaped to receive a supported article in contact therewith, and thereafter contacting the sacrificial ceramic fixture precursor to a molten reactive metal for a period of time sufficient to permit the sacrificial ceramic fixture precursor and the reactive metal to react together. The result is a reacted ceramic fixture, having the shape and dimensions required in the final fixture, comprising an open-cell solid foam of ceramic cell walls having an interconnected intracellular volume therebetween. The method further includes positioning the reacted ceramic fixture in relation to the supported article to produce a process assembly, and thereafter processing the process assembly as required for the supported article.
The sacrificial ceramic fixture precursor is preferably silica, and the reactive metal is preferably an aluminum-based metal. The resulting solid foam has alumina cell walls. Preferably, the ceramic cell walls comprise more than about 60 percent by volume, and most preferably from about 60 to about 80 percent by volume, of the open-cell solid foam.
The sacrificial ceramic fixture precursor is preferably provided by slip casting the sacrificial ceramic fixture precursor from a sacrificial ceramic slip material comprising particles of the sacrificial ceramic, and thereafter drying and firing the sacrificial ceramic fixture percursor. One advantage of the present approach is that the shape and dimensions of the sacrificial ceramic fixture precursor change very little during the contacting step. However, if there is some minor change, it may be easily corrected in the event that the sacrificial ceramic has a softening temperature. In that case, prior to the contacting step the sacrificial ceramic fixture precursor made of the sacrificial ceramic is heated to a temperature above the softening temperature, and the dimensions of the sacrificial ceramic fixture precursor are corrected while it is at a temperature above the softening temperature.
In one embodiment, after the step of contacting and before the step of supporting, metal is removed from at least a portion of the intrcellular volume of the reacted ceramic fixture to produce porosity therein. The porosity may be left as-is. Alternatively, at least some of the porosity may be filled with a filler material such as a filler metal or a surface sealant.
In a typical application, the supported article comprises a nickel-base superalloy. In positioning, the reacted ceramic fixture is usually, but not necessarily, contacted to the supported article. The processing involves heating the process assembly, including the fixture and the supported article, to a temperature greater than room temperature.
The process assembly of the present approach thus comprises a fixture having a body with a shaped portion thereof shaped to receive a supported article in contact therewith. The body comprises an open-cell solid foam of ceramic cell walls having an interconnected intracellular volume therebetween. A supported article is in contact with the shaped portion of the fixture body.
The present approach provides a method for processing a supported article using a ceramic or ceramic/metal fixture that has excellent properties over a wide temperature range, is dimensionally stable over a wide temperature range, and has excellent wear resistance. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.